When did you submit your representations ?
This thread shows that you were about to submit them in mid-January. There is no time limit in the LLA & TfL Act 2003 for responses to reps against a postal PCN, but there is a 56 day limit in the TMA 2004 for formal reps against an NtO or a postal PCN. Whilst there is no limit in the 2003 Act, one could argue that the limit in the TMA 2004 is clearly intended to prevent councils procrastinating and causing unfairness by their dilatoriness, and the same principal should apply to your case. Clearly their response is well over 56 days and you could argue this has caused unfairness to you when preparing an appeal to London Tribunals, as your memory has faded with time, and documents you may need at an adjudication may have got lost during the excessive time to respond.
Of course, there is no gurantee this approach will succeed at London Tribunals, but at least it would receive consideration. As you are probably aware by now, councils ruthlessly game the system to maximise their income. Are you happy to let them continue to do so ?